This application claims the benefit of German Application Serial No. 199 13 042.6, filed Mar. 23, 1999.
The invention concerns an adhesive for the treatment of textile reinforcement inlays for the production of textile-reinforced rubber products, in particular for a tire cord that is present in the form of an aqueous dispersion with a solids fraction of blocked isocyanate of 55-85 wt %.
In the production of fiber-reinforced rubber products, it has proved advantageous if an adhesive is used for the improvement of the adhesive strength between textile reinforcement inlays and the rubber. The use of such an adhesive, in particular in the area of tire cords and other high-stress composite materials with reinforcement fibers, is important. Especially for these fields of application, the use of resorcinol-formaldehyde-latex systems (RFL) for the bonding of synthetic fibers to rubber products is known from the state of the art. In accordance with the method, the procedure can be carried out in a one-step or in a two-step method. In the one-step method, an impregnation of the reinforcement element with a mixture of RFL and an adhesive is carried out.
In the two-step method, an impregnation of the reinforcement element is undertaken first with the adhesive and then the application of RFL is carried out in a second step.
From the state of the art, specially coordinated adhesives are also already known for such methods. These systems are based on a special formulation of an aqueous dispersion of blocked isocyanates in combination with RFL. In an overview article from xe2x80x9cRubber Chemistry and Technology,xe2x80x9d Vol. 58, p. 383-391, the corresponding adhesive systems and the methods for the production of fiber-reinforced rubber products are described. A special adhesive system is disclosed in U.S. Pat. No. 4,477,619.
From xe2x80x9cTire Technology International,xe2x80x9d 1994, page . . . to . . . , an adhesive system is known, which is sold under the name Grillbond IL-6(copyright) by the EMS Chemie AG in Switzerland. This system consists of an aqueous dispersion, which contains a 50% solids fraction of a blocked isocyanate. In comparison with the powders which are also known from the state of the art, this system has, in particular, the advantage that the aqueous dispersion is stable in storage and can be used immediately. This makes possible a simple handling and an economical execution of the method. At the same time, it has been shown that, in this way, neither an agglomeration nor a sedimentation of solids occurs, as is obviously the case in the preparation of powders with water.
The disadvantage with the previously described aqueous dispersion Grillbond(copyright), however, is that the solids fraction is relatively low and that for this reason, large quantities must be used, so as to guarantee a complete impregnation of the fibers.
Therefore, on the basis of this, the goal of the invention under consideration is to propose an adhesive system which has an increased solids fraction, in comparison to the state of the art, and with which a complete impregnation of the fibers is possible.
The goal is preferably achieved by the characterizing features of the invention. Advantageous embodiments and further developments of the invention will be apparent from the description provided herein.
Surprisingly, the applicant was able to show that it is possible to obtain an aqueous dispersion with a solids fraction of 55-85%, if the average particle size of the solids is  less than 2.0 xcexcm. Experiments carried out by the applicant have shown that in the aqueous dispersions of the state of the art, average particle diameters of over 3 xcexcm are present. By means of the dispersion proposed in accordance with the invention, wherein the average diameter of the solids particle is clearly below that of the prior state of the art, it is thus possible to increase the solids fraction to up to 85% without agglomerations or other disturbances occurring. The essential advantage of the aqueous dispersion according to the invention under consideration is that the work can now be carried out with lower quantities and thus with a more economical method. It has also been shown that the aqueous dispersion makes possible a complete impregnation of the reinforcement elements because of its uniform particle diameter. In this way, textile fiber-reinforced rubber products can be produced, which are clearly improved in their technical dataxe2x80x94that is, in particular, in adhesive strength, in comparison to the state of the art.
It is particularly preferred in the aqueous dispersion in accordance with the invention, if the average particle diameter is  less than 1.50 xcexcm, with particular preference,  less than 1.35 xcexcm.
By experiments which were carried out by the applicant, it was possible to determine that 80% of the solids particles have a diameter of 0.01-5 xcexcm. It is preferable, however, if the diameter is between 0.5 and 5 xcexcm. This shows that a more uniform particle size distribution and smaller particle sizes are present in the aqueous dispersion in accordance with the invention than was the case with the state of the art. The particle size distribution varies only within very narrow limits. The dispersion in accordance with the invention accordingly contains the solids in the form of a fine or very fine comminution.
From a material point of view, the invention comprises, with regard to the adhesives, all isocyanates which, in fact, have been used up to now in the state of the art for these application cases. The invention thereby includes isocyanates and the corresponding reaction products from the isocyanates with themselves to form dimeric or oligomeric products and those with blocking agents, to form partially or completely blocked isocyanates. In general, all commercially obtained aromatic isocyanates and aliphatic and cycloaliphatic isocyanates can be used. Special examples include the following:
Diphenylmethane-4,4- and/or -2,4-diisocyanate (MDI), raw and purified polymer MDI (PMDI), and all other MDI isomers, such as: 3,4-MDI, 2,2-MDI, and/or 2,3-MDI. 2,4- and/or 2,6-toluene diisocyanate and their oligomers, in particular, dimerized 2,4-toluene diisocyanate. 1-Isocyanato-3-isocyanatomethyl-3,5,5,-trimethylcyclohexane (isophorone-diisocyanate or IPDI), hexamethylene diisocyanate (HDI). Polyisocyanates in the form of polyisocyanate adducts, for example, polyisocyanate-containing groups from isocyanurates, uretdiones, biuret, urethanes, allophanates, carbodiimides, and/or oxadiazines.
Examples of blocking agents include the following:
Monophenols, for example, phenol, resorcinol, cresol, trimethyl phenols, tert-butylphenols, lactams, for example, Hcaprolactam, xcex93-valerolactam. Oximes, for example, methyl ethyl ketoxime (butanone oxime), methyl amyl ketoxime, and cyclohexanone oxime. Primary, secondary, and tertiary alcohols, glycol ethers, readily enol-forming compounds, such as acetoacetic ester, acetyl acetone, malonic acid derivatives. Secondary aromatic amines. Imides, mercaptans, and triazoles.
Wetting agents and/or dispersants are used as additives.
The adhesive in accordance with the invention is particularly suitable for textile reinforcement inlays, for example, made of polyester, polyethylene, polyamide, or rayon. The reinforcement inlays treated in this matter are used, in particular, for the production of tire cord, conveyor belts, V-belts, mechanical rubber parts, as well as composites.
The invention also concerns a method for the production of the previously described adhesive. The production of the adhesive in accordance with the invention is carried out thereby in a two-stage process. The procedure is such that in a first step, a preliminary dispersion of the adhesive is produced with water in the form of an aqueous dispersion with, for example, 50% solids content. In the further course, this preliminary dispersion is then subjected to a wet comminution, until the total material to be comminuted is water-thin and a viscosity of 5-150 mp, preferably 5-90 mp. The water-thin, comminuted material thus produced then permits the required further addition of solids, until the desired solids content of 55-85 wt % is attained. The wet comminution is thereby preferably carried out with a stirrer-ball mill.
If necessary, a dilution, for example, to 60% can again be produced by the further addition of water once a certain solids content, for example, of 70%, has established, if this should be necessary because of certain reasons for the production of very specific, selected rubber products.
Surprisingly, it has become evident that the adhesive system, produced as described in the preceding, has an excellent storage stability of at least one year. It was particularly surprising that with the adhesive composition in accordance with the invention, an addition of a thickener, as is required with compositions known from the state of the art, was not necessary. The settling behavior was thereby better without an additional thickener than with the formulations known from the state of the art.